Multipole rod set ion guides are known comprising four, six or eight parallel rods which are equi-spaced about a circular circumference. The rods are all maintained at substantially the same DC voltage. A two-phase RF voltage is also applied to the rods with adjacent rods being supplied with opposite phases of the RF voltage. The RF voltage applied to the rods causes a symmetrical effective radial pseudo-potential well to be generated within the space circumscribed by the rods. The radial pseudo-potential well causes ions to be confined radially within the ion guide. The ion guide may be maintained at a relatively high pressure and can result in a reduction in the ion radial density distribution due to collisional cooling of ions with background gas molecules. The multipole rod set ion guide may be arranged to confine, transport and focus ions in the presence of background gas. The known rod set ion guide may be used, for example, to couple an Atmospheric Pressure ion source to a mass analyser which must be maintained at relatively low pressure.
Another form of ion guide is known which comprises a plurality or stack of ring electrodes having apertures through which ions are transmitted in use. Opposite phases of a two-phase AC or RF voltage are applied to adjacent electrodes. The ion guide may comprise an ion tunnel ion guide comprising electrodes which have apertures which are all substantially the same size or diameter. Alternatively, the ion guide may comprise an ion funnel ion guide comprising a plurality of electrodes which have apertures which progressively decrease in diameter along the axial length of the ion guide.
Another form of ion guide is known which comprises a stack or a plurality of layers of intermediate planar electrodes. The plurality of intermediate planar electrodes are bounded by a single upper planar electrode on one side and a single lower planar electrode on an opposed side. Each layer of intermediate planar electrodes comprises two longitudinal electrodes. The two longitudinal electrodes in any layer are supplied with the same phase of a two-phase RF voltage. Adjacent layers of intermediate planar electrodes are supplied with opposite phases of the two-phase RF voltage. The RF voltage applied to the layers of intermediate planar electrodes causes a pseudo-potential well to be generated which acts to confine ions between the longitudinal electrodes within the ion guide in the horizontal radial direction. Voltages are applied to the upper and lower single planar electrodes in order to confine ions within the ion guide in the vertical radial direction.
The known ion guide comprising a stack of layers of intermediate planar electrodes and single upper and lower planar electrodes is particularly advantageous compared to other known ion guides in that various complex and efficient ion transport volumes or geometries can be provided that would be otherwise be very difficult to provide using a rod set ion guide or an ion guide comprising a plurality of ring electrodes.
Ion guides comprising a stack or plurality of layers of intermediate planar electrodes can be relatively easily designed so as to transportions along relatively convoluted or potentially complex ion paths. A further advantage of an ion guide comprising a plurality of layers of intermediate planar electrodes is that the shape and/or area of the ion confinement volume can be arranged to vary along the length of the ion guide. This enables the ion guide to effectively couple two components of a mass spectrometer which may have different ion-optical acceptance profiles.
According to an arrangement an ion guide comprising a plurality of layers of intermediate planar electrodes may be arranged so that two or more separate ion guides merge into a single ion guide. Alternatively, according to another arrangement an ion guide comprising a plurality of layers of intermediate planar electrodes may be arranged so that an ion guide divides into two or more separate ion guides.
The known ion guide comprising a plurality of layers of intermediate planar electrodes and single upper and lower planar electrodes suffers from the problem that collisions between ions and background gas molecules present within the ion guide may reduce the kinetic energy of the ions as they pass through the ion guide. This can have the effect of increasing the transit times of ions as they pass through the ion guide.